It is often desirable to selectively deposit a group IV semiconductor material such as silicon or germanium on semiconductor surfaces without depositing same on insulating surfaces.
For example, advanced complementary metal-oxide-semiconductor (CMOS) transistors are often fabricated using selective deposition techniques that grow epitaxially mono-crystalline semiconductor films only on the active areas of the transistors, commonly known as elevated source/drain structures.
Generally speaking, selective deposition takes advantage of differential nucleation during deposition on disparate materials. The precursor of choice will generally have a tendency to nucleate and grow more rapidly on one surface and less rapidly on another surface.
At the beginning of a nucleation stage, discontinuous films on insulating materials (e.g. oxides) have a high exposed surface area relative to merged, continuous films on semiconductor materials (e.g. silicon).
Accordingly, state of the art selective deposition methods report using Cl-comprising precursors (such as dichlorosilane), or supplying HCl in the reactor during the deposition process or a combination of both to achieve selectivity. Chlorinated precursors (such as dichlorosilane) or etchants (such as HCl or Cl2) are often used for further inhibition of the poorly nucleating film on the insulating material, as compared to the rapidly nucleating film on the semiconductor material. Germanium growth with GeH4 is also selective when H2 is used as a carrier gas, without the need of adding HCl.
A drawback of widely used silicon precursors like silane (SiH4), disilane (Si2H6) and trisilane (Si3H8) is that they are not selective towards insulator materials such as oxide and nitride, such that the silicon growth takes place both on exposed semiconductor materials and insulator materials.
Typically, a selective deposition process is tuned to produce the highest deposition rate feasible on the region covered by a semiconductor material while accomplishing no or substantially no deposition on the regions covered by the insulating material.
Advanced device manufacturing often requires a reduced temperature budget for the selective deposition steps. Under this new constraint, the known methods for selective deposition based on a thermal activated process are no longer feasible, on account of the unacceptable low deposition rates at low temperature.
Using dichlorosilane (Si2H2Cl2, DCS) or adding HCl should make the deposition selective. However, neither DCS nor HCl decompose at temperatures below 500° C. Therefore, at growth temperatures below 500° C. either no selectivity or simply no growth at all is observed. In particular, in the case of dichlorosilane, one monolayer of Si can be grown and then the reaction (growth) stops because Cl cannot be desorbed at the low reaction temperature.
In the case of germanium growth on a patterned semiconductor substrate comprising silicon areas and silicon-oxide areas, it is known that the growth is selective when germane (GeH4) is used as precursor and hydrogen (H2) as carrier gas in the deposition chamber. However, for temperatures below 350° C. germanium growth with GeH4 and H2 as carrier gas is not possible since germane does not decompose.
Digermane (Ge2H6) in H2 or N2 still decomposes at temperatures below 350° C. and can allow germanium growth, but the growth is not selective towards oxide, at least not for partial pressures of digermane higher than about 10 mTorr or higher than about 20 mTorr depending on the growth temperature for a total pressure in the reactor up to atmospheric pressure. This drawback is illustrated by the comparative test in FIG. 2A.
When Ge is grown on InGaAs and when using nitride spacers, a total loss of selectivity has been observed for Ge growth. In this case, indium outdiffusion from the surface to the nitride spacers might create a seed layer for Ge and then disable selectivity. This is illustrated by the comparative test in FIG. 3A.
Despite selective deposition of semiconductors being of considerable commercial importance for a variety of industrial applications, problems with respect to selectivity thus arise with known processes. Thus, there is a need for improved methods to selectively deposit semiconductor films onto semiconductor surfaces.